Papers by Author: Y.P. Ma

Abstract: Deposition processes of conventional diamond films and smooth fine-grained diamond films on cemented tungsten carbide inserts (SCMT120408-HR, 6wt. %Co) in the hot filament chemical vapor deposition (HFCVD) apparatus were investigated. A novel combined pretreatment of Murakami’s reagent, acid etching and microwave plasma decarburization in Ar-H2 gas was carried out. The adhesive strength of the films to the substrates, diamond-coated tool wear and surface roughness of the workpiece were further studied by turning glass fiber reinforced plastics (GFRP) cylindrical bars. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and Raman spectroscopy were employed to characterize the substrates and diamond films subjected to different pretreatments and deposition processes. Optical microscope and surface scanning profilometer were utilized to monitor the evolution of tool wear and surface roughness of the workpiece respectively during the cutting test. The experimental results showed that the novel combined pretreatment was more effective in enhancing the adhesive strength of diamond films to the substrate. Fine WC interlayer generated during the initial stage of the diamond film deposition played an important role in improving the adhesion. Diamond coated tools subjected to such pretreatment demonstrated up to 6 times longer tool life than uncoated ones. Varied parameter deposition process produced smooth fine-grained diamond films, which led to remarkably lower surface roughness of the workpiece in turning GFRP

Abstract: Despite great advancements in diamond thin film growth and deposition techniques, determination of the residual stress and Young’s modulus for diamond films has continued to be a challenge. The bulge test is a potentially powerful tool for characterizing the mechanical properties of diamond film. In a bulge tester, pressure is applied on a thin membrane and the out-of-plane deflection of the membrane center is measured. The Young’s Modulus and the residual stress are simultaneously determined by using the load-deflection behavior of a membrane. By means of electron-enhanced hot filament chemical vapor deposition (HFCVD), a diamond film was deposited on silicon slice (100), and the free-standing window sample of diamond thin films was fabricated by means of photolithography and anisotropic wet etching. The deflection of the membranes is measured using a laser interferometry system. The elastic modulus and residual stress were measured using a self-designed bulge equipment. In addition, the distortion of diamond thin films under different pressure was simulated using finite element analysis and the contrast was made with experimental data. The research indicated that the Young’s Modulus of diamond thin films is 937.8GPa and the residual stress is -10.53MPa. The elastic modulus and the residual stress coincide with the report in the literature and the value tested by X-ray diffraction, respectively. This method uses a simple apparatus, and the fabrication of samples is very easy, and it has provided an effective means for precise measure the mechanical properties of other thin films.

Abstract: Nanocrystalline diamond film with smooth surface and uniform grains was deposited successfully on Co-cemented carbide using the bias-enhanced hot filament chemical vapor deposition (HFCVD). The surface morphology and chemical quality of film were estimated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman spectroscopy. Comparative experiments of tribological and wear performances of conventional and nanocrystalline diamond films were carried out by pin-on-disc tester. The research results show that nanocrystalline diamond film with good tribological performance and high quality can be deposited by regulating the deposition parameters on Co-cemented carbide. The film not only has high adhesive strength but also has smooth surface, low surface roughness, low friction coefficient. The work done in this paper provide the wide application of diamond on complex shape tools, drawing dies and other wear resistant device with experimental reference.

Abstract: Improving adhesion and surface roughness of diamond films on WC–Co substrate is the key factor of the widespread application of diamond coated tools. A new pretreatment method has been performed for smooth Co-cemented carbide inserts in order to lower the surface roughness of diamond films under the premise of good adhesion between diamond films and substrates. The effect of the new pretreatment on the adhesion of the diamond films is investigated. Research results show that the boronization pretreatment can effectively suppress cobalt diffusion to the surface and avoid catalytic effect of Co at high temperature. This new pretreatment can avoid the surface roughening of inserts and ensure the deposition of smooth diamond films. Investigation shows that the optimum boronization compounding is a powder mixture of 70%B4C+15.5%KBF4+1.5% La2O3+13%Na2CO3. Adhesion between substrates and diamond films is evaluated by Rockwell A indentation tests and the cutting performance of the diamond-coated tools is investigated by the cutting tests. Diamond films on smooth cemented carbide inserts with cobalt boride interlayer have high adhesive strength and low surface roughness. Diamond-coated tools with boronization pretreatment have a 5-fold increase in tool life compared with untreated ones.

Abstract: Nano ceramics possesses excellent mechanical property and physical characteristics in contrast to conventional engineering ceramics, so it has tremendous application prospect. Adopting ultrasonic composite processing we describe the influences of grinding speed, grinding depth, wheel granularity and no-spark grinding times on the surface roughness of nano ZrO2 ceramics. By means of SEM and AFM the surface character and critical ductile grinding depth of nano ZrO2 ceramics in the condition of conventional and ultrasonic grinding are also discussed. At last, the residual stress of surface and crystalline phase transformation under the condition of conventional grinding and ultrasonic vibration grinding were analyzed by X-ray diffraction. The research indicated that ultrasonic vibration grinding could obtain nano finished surface with high efficiency. The residual stress of nano ZrO2 ceramics surface is determined much by different grinding styles.

Abstract: The crack extension course and ductile removal mechanism of nano ZrO2 ceramics were analyzed in this paper. On the basis of contrast tests with or without ultrasonic vibration, the influences of critical ductile grinding depth on grinding forces and surface quality were studied by dynamometer, SEM and AFM in different grinding condition. The reason for the increase of the critical grinding depth was discussed based on the analysis of grinding force and ultrasonic vibration course. At last, the formation mechanism of surface topography observed by AFM in ductile domain was analyzed. The research indicated that ultrasonic machining could obtain nano finished surface with high efficient.

Abstract: Due to the large difference in thermal properties of the resin and the abrasive grits, the laser-assisted dressing technology is of great interest for grinding wheel preparation. From a viewpoint of thermal induced material removal mechanism, a numerical method was presented to simulate the laser dressing process. The numerical simulation results could reveal the relations between the laser parameters and the groove formation. The overlap coefficient was introduced for practical application. Simulation for the variation of the number of active grinding points was also made for non-uniform wheel topography. Based on the numerical modeling, the suitable grinding^wheel surface topography can be achieved through the laser-assisted dressing technology. The comprehensive researches on the laser-assisted dressing process control, grinding wheel topography reconfiguration by 3D laser scanning technology and analyses of grinding temperature were made. A series of grinding tests with the laser-assisted dressed grinding wheel and mechanically dressed grinding wheel were conducted for comparison. The results proved the feasibility of laser-assisted dressing for resin bonded superabrasive grinding wheels and revealed the importance of choosing appropriate laser dressing parameters.